Kef Q100 Speaker Drive Unit Testing

As with the Kef HTS3001SE I tested, I ordered a set of the Kef Q100 Bookshelf speakers in order to remove and review the raw Q100 driver itself.  I really wanted to see how this coaxial design performed.  Zaph had already tested this one but I wanted to do Klippel LSI testing on it to see how the suspension performed.  He actually mentioned this in his writeup and I thought it would be cool to provide the results.  Of course, since I had it on the test baffle I did some other standard measurements as well.  The one I was interested in, but didn’t perform on the HTS3001SE driver was tweeter frequency response performance with movement of the woofer.  I don’t necessarily have an easy way to test this so I did something a bit different: I used a 9v battery to statically ‘fix’ the woofer either in the coil out or coil in position and measured the response.  I then compared this to the woofer at rest performance of the tweeter and did a direct comparison.  This is discussed further below.

On to the testing…


Up first, obligatory pictures:


IMG_5291  IMG_5288 IMG_5289 IMG_5290

This driver is quite the little beast.  A very large motor and pretty substantial surround make this one of the largest 5.25″ drivers I’ve personally seen.  Although I didn’t weigh it, it is fairly heavy due to the woofer’s ferrite magnet as opposed to neodymium.  This results in large and heavy.  I can’t exactly measure the voice coil but comparing it to the tweeter assembly, it appears to be a few mm larger in radius so I’d estimate VC diameter at roughly 55mm.  It is best to rear mount this driver given the very tall surround at approximately 12mm, but for the purpose of my test I front mounted it.

If you look at the ‘Tangerine’ waveguide/lens/whatever you want to call it, you’ll notice there’s actually a phase plug on the tweeter.  The HTS3001SE does not have this.

For those who want to read about the Tangerine waveguide, click this link (PDF format).  There’s also discussion on the radial ribbing of the other Uni-Q cones, which this driver doesn’t employ.



Raw Driver Physical Measurements

First off, given this isn’t sold as an individual driver, I have taken my own measurements.  These are rough measurements taken with my not-so-recently calibrated calipers, but should be good within +/-1mm.

Outer Diameter 143 mm
Mounting Diameter 120 mm
Mounting Depth   83 mm
Effective Piston Diameter* 109 mm
Effective Piston Diameter**   60 mm
Flange Thickness 0.34 mm
Mounting Tab Thickness 0.65 mm
*Half surround to half surround; including space consumed by coincident tweeter.
**Half surround to half surround; NOT including space consumed by coincident tweeter.

Test Results

To make things a bit easier to manage, I’ve broken down the test results in to two sections:

  1. Woofer Testing
  2. Tweeter Testing

Part I: Woofer Testing

Woofer Thiele-Small Parameters and Impedance

Note:  When determining the full suite of T/S parameters, the effective diameter of the driver is needed to calculate Vas, Bl, etc.  Most of the time this can simply be measured by measuring the diameter of the driver from half-surround to half-surround since the motor must control the entire cone area.  However, in this case, the entire cone does not move.  Therefore, the effective diameter (and resulting Sd) is not the entire diameter of the driver.  The effective diameter here is determined by subtracting the static tweeter assembly from the overall effective diameter of the woofer.  See physical measurements section above for all values.

Electrical Parameters
Re 3.09 Ohm electrical voice coil resistance at DC
Le 0.256 mH frequency independent part of voice coil inductance
L2 0.427 mH para-inductance of voice coil
R2 3.39 Ohm electrical resistance due to eddy current losses
Cmes 318 µF electrical capacitance representing moving mass
Lces 18.82 mH electrical inductance representing driver compliance
Res 29.82 Ohm resistance due to mechanical losses
fs 65 Hz driver resonance frequency
Mechanical Parameters
(using add. mass)
Mms 12.834 g mechanical mass of driver diaphragm assembly including air load and voice coil
Mmd (Sd) 12.108 g mechanical mass of voice coil and diaphragm without air load
Rms 1.352 kg/s mechanical resistance of  total-driver losses
Cms 0.467 mm/N mechanical compliance of driver suspension
Kms 2.14 N/mm mechanical stiffness of driver suspension
Bl 6.35 N/A force factor (Bl product)
Loss factors
Qtp 0.365 total Q-factor considering all losses
Qms 3.878 mechanical Q-factor of driver in free air considering Rms only
Qes 0.402 electrical Q-factor of driver in free air considering Re only
Qts 0.364 total Q-factor considering Re and Rms only
Other Parameters
Vas 3.6615 l equivalent air volume of suspension
n0 0.241 % reference efficiency (2 pi-radiation using Re)
Lm 86.02 dB characteristic sound pressure level (SPL at 1m for 1W @ Re)
Lnom 87.14 dB nominal sensitivity (SPL at 1m for 1W @ Zn)
Sd 74.46 cm² diaphragm area

q100 impedance

Woofer Large Signal Analysis with Klippel’s LSI Module

Displacement Limits thresholds can be changed in Processing property page
X Bl @ Bl min=82% >4.2 mm Displacement limit due to force factor variation
X C @ C min=75% 1.9 mm Displacement limit due to compliance variation
X L @ Z max=10 % 2.8 mm Displacement limit due to inductance variation
X d @ d2=10% 17.1 mm Displacement limit due to IM distortion (Doppler)

q100 bl q100 bl symmetry q100 kms q100 kms symmetry q100 cms q100 le q100 li

Woofer Frequency Response

Measured at 2.83v/1m.  Stitched with a nearfield measurement at approximately 500hz.

Kef Q100 Drive Unit (Woofer) 0 30 60

Woofer Harmonic Distortion

kef q100 woofer FR HD 96dB

Part II: Tweeter Testing

Small Signal Parameters:

Electrical Parameters
Re 2.84 Ohm electrical voice coil resistance at DC
Le 0.018 mH frequency independent part of voice coil inductance
L2 0.011 mH para-inductance of voice coil
R2 0.5 Ohm electrical resistance due to eddy current losses
Cmes 110 µF electrical capacitance representing moving mass
Lces 0.28 mH electrical inductance representing driver compliance
Res 1.09 Ohm resistance due to mechanical losses
fs 902.6 Hz driver resonance frequency
Loss factors
Qtp 0.491 total Q-factor considering all losses
Qms 0.678 mechanical Q-factor of driver in free air considering Rms only
Qes 1.769 electrical Q-factor of driver in free air considering Re only
Qts 0.49 total Q-factor considering Re and Rms only

Tweeter Frequency Response

Kef Q100 Drive Unit (Tweeter Only) 0 30 60

Tweeter Harmonic Distortion

Kef Q100 Tweeter FR HD 96dB

Tweeter Response vs Woofer Position

I thought it would be interesting to see how the position of the woofer cone impacts the frequency response of the tweeter.  This matters when you’re listening to music and isn’t captured by a standard sine sweep.  To measure this performance I simply connected a 9v battery to the woofer’s terminals in positive polarity, then negative polarity which resulted in an approximate +/-3mm shift in cone direction.  I ran a sine sweep over the tweeter while the woofer was a) at rest, b) fixed out, and c) fixed in.  The pictures below show illustrate this.

Woofer at rest:

Q100 Woofer At Rest

Woofer fixed out:

Q100 Woofer Out

Woofer fixed in:

Q100 Woofer In

The following results are of the three positions discussed above overlaid on one another.  The lines are labeled per the woofer position.

Note: The SPL level is not absolute here.  I performed the test at the same volume level throughout but it is not intended to reference any set test paraemter such as 2.83v/1m or 1w/1m.

Kef Q100 Drive Unit Woofer Displacement on Tweeter Response Example


If you have any specific questions or you have feedback on the performance of this driver, feel free to post to my page.

If you like what you see here and what to help me out, there’s a Paypal Contribute button at the bottom of each page.  Every little bit does help.  Remember, I don’t get paid a dime to do this stuff.  I do it on my own for the love and entertainment but it is nice not to have to pay out of pocket to purchase things to test or gear to test with.  I’m looking to buy a new high-SPL capable mic for subwoofer testing so any donations in the near future will go toward that.





EDIT, 01/16/2013:

I was asked for a picture of the crossover that comes with the Q100 speaker.  Here it is:





Update 01/26/2013:

I’ve been using these Kef Q100 speakers (driver and cabs) as my reference/experimenting setup with dolby, L7, etc recently and I can say unequivocally, these are by far the best set of speakers I’ve heard in their respective size range. The imaging and soundstage along with vocal acuity is incredible. When I first fired them up I was extremely impressed. Walking around the speakers, there is no dramatic drop in response; the sound power response is excellent.

I had every intention on selling these to get funds back but at this point I’m making every effort to not have to sell them. I’d like to make them my new reference setup. A sub to pick up below 50hz mated to these would make a very potent and worthwhile setup rivaling many tower based setups I’ve heard. And this is coming from the guy with a set of Zaph ZRT 2.5’s. the cool part is its extremely portable so I can take it to meets and demo for others.

Kef really nailed this speaker. For its price I have yet to find anything I think could beat it.

12 thoughts on “Kef Q100 Speaker Drive Unit Testing

  1. Great writeup. Thank you.
    Do you happen to know the values of the crossover components?
    How did you get the driver out of the box for Pete’s sake?

  2. Hi, Erin.

    First, Great job here on KEF drivers ! I was just wondering about a +5 dB bump i found in Q100’s frequency response from 3K to 16K on this site I’m wondering if it’s the cross-over action. Could you please measure the frequency response of the whole speaker ? Thanks anyway !

  3. Excellent analysis on all 3 of these driver units (Q100, R300 and LS50). Would it be possible to get the same level of detail in the cabinet design of each of these?

  4. Hello,

    I’m a bit late but just purchased the R series drivers from a R600. Don’t believe they are different from what’s in the R300 that you were requesting in any way. I’d be happy to send 1 to you if you’re still interested?

    When I purchased my Q700 from KEF for my 1st home theater I didn’t realize 2 of the woofers didn’t have drivers. Not a fan of the ABR and considering putting 2 real drivers in their place on each tower. The JL Audio 6.5 w3 is 1 of the few I’ve seen that’d I’d like to put in them. Just wondering if putting those, KEF, or any others would be worth it? Mainly in part for wanting to replace the Q700 for the KEF LS50. Would really appreciate your advice


    How did this driver compare vs other brands in the same price range?

  5. 2 questions :
    1) The on-axis tweeter response is no where flat, but the 30 deg one is pretty flat. In’t that noticeable in listening on axis ?
    2) Regarding the woofer excursion impact on tweeter response. Were you able to determine a threshold where the woofer movement won’t impact the tweeter response. A high pass on this driver can help address that…


  6. Great detailed information about this driver! It show information I have no idea how to read but I did save it in case I need it for my little project.

    I bought two damaged X300A Wireless KEF speakers with badly damaged Uni-Q drivers (totally ripped). I am now looking onto replacing those but am having a hard time finding the exact model (I need the SP1647). Therefor I would like to ask you if you have any idea to differences between the SP1647 I need and the SP1587 you took from the Q100. Would swapping those be a viable option? I’m a handy guy but not into the exact details of sound/speaker configurations and all the tiny details that come with proper speaker building.

    Would love to hear your reply/view on this. In case you’d like here two links to my album with some pics:

  7. This is a truly marvellous piece of work!

    For many years I have appreciated the natural sound and lack of criticality in listening position offered by the Uni-Q “point source”. I already have 3 generations of Kef Q speakers and in many ways these look like a very interesting development! I currently have a Q15, Q35.2 and Q1. I really like the floor standing Q35.2 format, but the cone on one has recently failed in a rather spectacular fashion and the other does not look too good either. To my astonishment the failed one sounds OK though it obviously lacks any bass. I am having difficulty deciding what to replace them with with no affordable floor stander available: Q100 or Q300 perhaps?

    The use of aluminum for both drives is new for Uni-Q I think, but E J Jordan championed this first in around 1960. I remember listening to a Jordan Watts module speaker about forty year ago which reproduced male speech in a more lifelike manner than perhaps any other loudspeaker I have ever heard. Aluminium also features in Ted Jordan’s latest Eikona II full range deign, but they look too expensive to me and nobody really sells a properly finished complete speaker. I am not convinced by the claimed advantages of a full range unit.

  8. One thought on the Thiele-Small parameters is that the physics is about the area of the cone so the radius given should be that equivalent to the effective area of the cone.

    • It is. If you read the paragraph regarding the woofer portion of this driver I noted that Sd provided is based on the actual woofer area:
      “Therefore, the effective diameter (and resulting Sd) is not the entire diameter of the driver. The effective diameter here is determined by subtracting the static tweeter assembly from the overall effective diameter of the woofer.”

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